This invention relates to electrical contact coatings for silicon and particularly to electrostatically bondable coatings which will be useful in fabricating pressure transducers and which will provide a good electrical contact surface for the silicon of the transducers. Some pressure transducers are constructed by sandwitching an etched silicon diaphragm between silicon plates with a thin Pyrex glass layer separating the plates from the diaphragm so as to provide the necessary materials for electrostatic bonding of the plates to the diaphragm and for electrically isolating the plates from the diaphragm. This invention particularly relates to the provision of contact coatings for such pressure transducers and more particularly to contact coatings which must provide a contact surface for spring loaded plunger type contacts.
In U.S. patent application Ser. No. 594,585, entitled "ELECTRICAL FEEDTHROUGH MEANS FOR PRESSURE TRANSDUCER", filed by Paul B. Binder and Paul M. Kroninger, Jr. on Mar. 29, 1984, there is disclosed a means providing a bulkhead feedthrough using a spring loaded plunger type contacts to make electrical connection to a silicon pressure transducer of the structure described above. In the arrangement shown in that application, the areas on the silicon where electrical contact is made are shown as being coated with a thin film of metal, such as aluminum, for example. It has been found, however, that such coatings are not as durable as is desired. It is believed that contact by the spring loaded plunger causes the metal coating to wear off. This appears to be due to the scrubbing action of the contact on the surface as a result of vibration of the transmitter while it is in use. Also, since it is necessary to test the transmitters before final assembly and that testing quite naturally includes connection in a test circuit by the use of spring loaded contacts similar to those used in the pressure transducer assembly itself, the testing itself causes some wear on the metal coating of the contact area. Still further problems can arise from the thin metal coatings. For example, the coating may tend to disappear due to adverse conditions as the transducer is processed through the steps necessary in its construction. This disappearance may, for example, occur because the metal coating becomes the sacrificial electrode of an electrolytic pair. The adverse conditions which must be considered also include the heating of the transducer as is necessary in some of the steps of the process, such as the electrostatic bonding. In view of these requirements, there were set up the following criteria as being desirable for the coating of the contact areas of the silicon transducer:
1. Low electrical resistance PA1 2. Abrasion resistance PA1 3. Corrosion resistance PA1 4. High temperature resistance PA1 5. Electrostatically bondable to Pyrex 7740 glass.
It is known that platinum silicide provides a contact coating for silicon which meets the first four criteria set forth above. It is also known that electrostatic bonding of silicon and borosilicate glass has developed as the preferred method for bonding the elements of silicon transducers because of its firm bond and also due to the fact that it does not require any great increase in size of the final package as occurs when a glass-to-metal seal is made. Therefore, one of the necessary characteristics for the contact material is that it be electrostatically bondable, as indicated above. If it posseses this characteristic the coating can be laid down over the entire diaphragm and the entire surface of the silicon plates during manufacture of the transducer. This approach simplifies the manufacturing procedure, for masking is then not required, U.S. Pat. No. 3,397,278, issued to D. I. Pomerantz on Aug. 13, 1968 describes the electrostatic or anodic bonding procedure for several materials including silicon and borosilicate glass. It is an object of this invention to provide a contact coating for silicon which has resistance to abrasion, corrosion, and high temperature resistance as well as having a low electrical resistance and the ability to be electrostatically bondable with Pyrex 7740.